Neutralization of inductive interference



Patented July 27, 1926.

UNITED STATES 1,593,640 PATENT OFFICE.

HOWARD I. TRUEBLOOD, 01 1301335 FERRY, NEW YORK, ASSIGNOR TO AMERICANTELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

NEUTRALIZATION OF INDUCTIVE INTERFERENCE.

Application filed Kay 18,

tentials are neutralized they interfere with the reception of signalstransmitted over the communication circuits, the degree of interferencedepending upon the relative magnitudes of the two currents. Further- 2more, the interference may take the form of an actual breakdown of somepart of the communication circuits. Various means have been devised forthe neutralization of interfering potentials and some of these meanshave been used. One form of such interference reduction means which iscommonly known as a compensating transformer is disclosed in the patentto J. A. Barrett, No. 940,658, dated November 23, 1909. Suchneutralizing transformers are multiple-winding. transformers in which acertain proportion of the windings are used as primaries and theremainder as second aries. The primary windings are connected withcertain conductors of a group of communication conductors which have setup therein an interfering potential by some source extraneous to thecommunication circuits. Similarly each secondary winding is connectedwith one of the other conductors of the communication circuits, whichalso have an interference potential created in them. The windings are sopoled that the current flowing through the primary windings will inducea potential across the secondary windin which is substantially equal andopposite in phase to the potential set up in each of the conductorsconnected with the said secondary windings. This equality may besubstantially obtained by using suflicient primary conductors. Theseprimary conductors, however, are grounded, usually by means of simplexcoils, one for each air of wires, at suitably chosen points outsi e ofor near the ends of the exposed 1924. Serial No. 713,043.

section of the communication circuits. Owing to the fact that theseprimary conductors are grounded and also due to the required method ofgrounding they cannot be used for grounded telegraph operation. Since itis common practice to use long communication circuits for both telephoneand telegraph purposes, it will be apparent that the sacrificing of thetelegraph circuits means a considerable reduction in the volume oftrafiic that can be handled and loss of revenue. In practice, the numberof primary conductors necessary for the neutralization of interferenceis a relatively large fraction of the total number of conductors, thisbeing necessary because the ciliciency of neutralization requires theimpcdance of the circuits external to the primary transformer windingsto be small compared with that of the windings themselves; Thisinvention consists in the provision of a method and means whereby a highdegree of neutralization may be effected through the use of relativelyfew rimary conductors.

Other and further ob ects of this invention will be apparent from thefollowing description when read in connection with the attached drawing,of which Figure 1 shows a form of the invention in which one air ofprimary conductors may be utilize in neutralizing the interference setup m any number of the other communication c1rwits and Fig. 2 shows anarrangement whereby the interference may be substantially neutralized,not only in the communlcation circuits other than the primary c rcuitbut also within the primary circuit 1tself.

In Fig. 1, the source of disturbance is represented by a three-phase wetline which is connected by the trans ormer 15 with a source of power. Land L represent signaling or communication circuits, which by theconnection of well known types of terminal equipment may be adapted forthe simultaneous transmission of telephone and telegraph signals. Thecircuit L serves as the primary circuit of the neutralizing arrangement,and L represents a secondary circuit in which the interfering potentialis to be neutralized. It should be pointed out that although only onesecondary circuit is shown, any number of such circuits might beconnected with the neutralizing arran ement. The amount of energysupplied y the amplifying apparatus, which will be describedhereinafter, will, of course, depend upon the number of secondarycircuits. T e primary circuit 1L comprises the conductors 1 and 2, whichhave bridged across them the retardation coils 5 and 6, which arelocated outside of or near the ends of the exposed section, of thecommunication system. The midpoints of the coils 5 and 6 are grounded sothat between the ends of the exposure the conductors 1 and 2 areefi'ectively connected in parallel to ground. Also connected withconductors 1 and 2 are the windings 7 and 8 respectively, whichconstitute the primary windings of a transformer. The windings are sopoled that when current flows through both in the same direction, theireifect will be cumulative in setting up a potential in winding 9. Thesecondary winding 9 is connected with the potentiometer 10. The windings7 and 8 are designed to have low impedance so as not tointroduce'serious transmission loss in the circuit L by arranging themso that they are practically non-inductive to currents in oppositedirections through them, such as occurs when telephone currents aretransmitted over the line L,. The potentiometer 10 is connected with theinput side of the vacuum tube 11 by connecting one terminal with thegrid and the other with the filament thereof. The outputor plate circuitof the tube contains the primary winding 12 of the compensatingtransformer, the secondary windings 13 and 14 thereof being connectedserially with the conductors 3 ..;and 4 respectively of the line L,,.

The manner in which the system of Fig. 1 operates for the substantialneutralization of interference is as follows: When a field is set up bycurrent flowing over the conductors of the power line as, for example,current from transformer 15 to a fault, such as a ground upon one of theconductors, with a return through the earth potentials will be set up inthe conductors of the signaling circuits L and L These potentials willtend to produce a flow of current over the conductors of the saidcircuits which, as is well known, will be detrimental to the signalstransmitted as, for example, the teegraph signals carried by thesuperimposed telegraph circuits. Since the primary circuit L 18 groundedat the points where the retardation coils 5 and 6 are located, currentsto ground will flow therethrough. For purpose of description, these maybe assumed to be in the direction represented by the arrows. In likemanner a potential will be set up in each of the conductors 3 and 4of'circuit L which will tend .to cause current to flow in each in thedirection shown by the arrow. The flow of the dist-urblng currentsthrough the windings 7 and 8 of conductors 1 and 2 will induce apotential across the winding 9, which may be applied to the grid of thevacuum tube 11. The magnitude of this applied potential may, of course,becontrolled by the proper adjustment of the potentiometer 10. Thecurrent flowing through the primary winding 12 of the compensatingtransformer, which is in the plate-filament circuit of the tube 11, willbe controlled by the grid potential, which, as we have seen, iscontrolled by the potential created by the current flowing throughconductors 1 and 2. The windings 12, 13 and 14 are so arranged that thepotentials induced across windings 13 and 14 by the output current ofthe amplifier will cause a potential in each of the conductors 3 and 4which will be opposite to that created by the interfering potential setup by the power line. .By properly adjusting the otentiometer 10 theinterfering po tentials in the conductors of the circuit L and all othercircuits that might be connected with other secondary windings similarto 13 and 14, may be substantially neutralized.

The arrangement shown in Fig. 2 diflers structurally from that shown inFig. 1 in several ways. In Fig. 2 the potentiometer 10 is connectedbetween the midpoint of the retardation coil 5 and ground. Since thedisturbing currentset up by the power line flows, at any instant,through both conductors 1 and 2 in the same direction, as illustrated bythe-arrows, they will combine and flow through the potentiometer 10 toground. The potential applied to the grid of the vacuum tube 11 may becontrolled by varying the setting of the potentiometer 10. It willfurther be seen that the compensating transformer of Fig. 2 comprisesnot only the rimary winding 12 and the secondary windings 13 and 14connected with the conductors of the circuit L (as in Fig. 1) but alsothe windings 16 and 17, which are connected with the conductors 1 and 2of the primary circuit L. It will be apparent that when the disturbinpower line creates a potential which wil cause a current flow over theconductors of the circuit L, which, for illustration, may be assumed inthe direction represented by the arrows, a potential will likewise beinduced in the conductors of the circuit L,,, which will tend to cause acurrent flow in the direction represented by the arrows. The flow ofcurrent to ground from the midpoint of the retardation coil 5 in circuitL,, which current will traverse the potentiometer 10, will control thepotential applied to the grid of the vacuum tube 11. By varying thesetting of the potentiometer the magnitude of the current in thelatefilament circuit, which includes the primary winding 12, may also becontrolled. The windings of the compensating transformer are so arrangedthat the potentials induced in the windings 13 to 17 inclusive, willoppose the potentials set up in the same circuits by the disturbingpower line. It will thus be seen that by the proper adjustment of thepotentiometer 10 the magnitude of the compensating potentials may bemade substantially equal to the disturbing potentials which they areintended to neutralize. Neutralization of the disturbing potentials inthe conductors 1 and 2 of the line L will be complete except for thesmall residual potential necessary for the proper functioning of theamplifier.

The amplifier is shown in Figs. 1 and 2 in schematic form, and otherforms may be used without departing from the scope of this invention.If, for example, the frequencies involved are low, it may be desirableto use an amplifier having a plurality of amplifying elements certain ofwhich amlify the potential, and others the current. guch forms ofamplifiers are wel known .in the art, and may be substituted for thesimple type shown. In like manner other well known forms of amplifiersmay be used depending upon the requirements of par-' ticular cases ofinterference.

Although this invention has been disclosed as embodied in a particularform it is apparent that it is not so limited but is capable ofembodiment in other forms without departing from the spirit and scope ofthe appended claims.

What is claimed is 1. In an inductive interference neutralizing system,the combination with a power circuit, of a plurality of signalingcircuits having interfering potentials created therein by the said powercircuit, impedance coils bridged across the sides of one of saidsignaling circuits and having their midpoints grounded, an amplifier,means to impress upon the said amplifier a controllable part of theinterfering potential set up in said grounded signaling circuit, andmeans to impress upon the other of said signaling circuits the amplifiedpotential, the mode of impressing being such as to neutralize theinterfering potential created therein by the said power circuit.

2. In an inductive interference neutralizing system, the combinationwith a circuit having an interfering potential set up therein by anexternal source of interference, impedance coils bridged acrossthe sidesof the said circuit, the said coils having their midpoint grounded, apotentiometer connected with one of the said grounding connec tions, anamplifier having its input connected with the said potentiometer and acom pensating transformer having a primary and a plurality of secondarywindings, the said primary being connected with the output circuit ofthe amplifier, and each secondary winding being connected with one ofthe sides of the said circuit having an interfer- I ing potentialcreated therein so that the said interfering potential will beneutralized by a potential created in the secondary windings bythecurrent flowing through the primary in the output side of the amplifier.

3. In an inductive interference neutralizing system, the combinationwith a plurality of circuits, each having an interfering potentialset uptherein by an external source of interference, impedance coils bridgedacross the sides of one of said circuits, each of said coils having itsmidpoint grounded, an amplifier, means to effectively apply to the inputof the said amplifier a controllable part of the potential resultingfrom the interfering currents in both sides of said one of saidcircuits, a compensating transformer having a primary winding connectedwith the output of the amplifier and having secondary windingsindividual to and connected with each side of the other of saidcircuits.

4. In an inductive interference neutralizing system, the combinationwith a plurality of circuits, each having an interfering potential setup therein by an external source of interference, impedance coilsbridged across the sides of one of said circuits, each of said coilshaving its midpoint grounded, an amplifier, means to effectively applyto the input of the said amplifier a controllable part of the potentialresulting from the interfering currents in both sides of said one ofsaid circuits, a compensating transformer having a primary windingconnected with the output of the amplifier and having secondary windingsindividual to and connected with each side of all of said circuits.

5. In an inductive interference neutralizing system, the combinationwith a plurality of circuits, each having an interfering potential setup therein by an external source of interference, impedance coilsbridged across the sides of one of said circuits, each coil having itsmidpoint grounded, an amplifier, means connected with one of thegrounding connections to apply to said amplifier a controllable part ofthe potential created by the interfering current in both sides of thesaid one of said circuits, and means to couple the output of the saidamplifier with each side of all of said circuits, said coupling being sopoled that the potentials created by the amplifier in each side of saidcircuits will oppose and substantially neutralize the interferingpotentials therein.

6. In an inductive interference neutralizing system, the combinationwith a circuit having an interfering potential set up therein by anexternal source, an impedance bridged across the sides of the saidcircuit at or beyond each end of the said source, each impedance havingits midpoint grounded so that the interfering currents will flow j overthe two sides in parallel to ground, an

amplifier, means connected with one of the grounding connections toapply to the said amplifier a controllable part of the potential createdby the interfering current in both sides of the said circuit, and acompensatin transformer having its primary connecte with the output ofthe said amplifier and having a plurality of secondary windings,

each' individual to and serially connected with the sides of thesaid"circ-uit at a point upon the said circuit between the twoimpedances.

In testimony whereof, I have signed my name to this specification this12th day of May, 1923. x

- HOWARD M. TRUEBLOOD.

